Microphone amplifier



Feb. 9,1937. O WQHLFARTH 2,069,868

MICROPHONE AMPLIFCIER'I Filed March 2, 1935 INVENTOR OTTO WOHLF ARTH ATTQRNEY.

V Patented Feb. 9, 1931 UNITED STATES ant-9,868

MICROPHONE AMPLIFIER Otto Wohli'arth, Berlin, Germany,,assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany Application March 2, 1935, Serial No. 8,969 In Germany February 1, 1934 5 Claims.

This invention is concerned with an amplifier with mains-connection means in combination with a microphone. According to this invention,

, the operating current for the microphone is de- 5, rived from arectifier by way of a filter or smootha ing means, the capacities of the latter consisting of electrolytic rectifiers.

This invention shall-be explained in more detail by reference to the accompanying drawing. The amplifier comprises the two multi-grid type tubes I and 2, for which the working potentials are derived from alternating current mains by way of the main switch 3 and the primary winding 3a 7 of transformer 4. The" heating current for the [5 tubes is taken from the winding 5, andthe plate potential "from the winding 6 through the rectifier tube 1 and the low-pass filter 8.

. According to this invention, the operating D. C. for the microphone 9 is derived from the winding III of the mains-connected transformer by way of the rectifier Ii and thelow-pass filter. The recin'fier H is preferably of the so-called dry kind, say, a selenium or a cuprous-oxide rectifier. .7 The shunt capacities of the filter chain l2 con sist of electrolytic condensers. Ithas been ascertained that a size of 100 mfd. each sufilces to insure adequate smoothing of the rectified voltage to be supplied'to the microphone, sothat disturbing (background) noises will-not be perceivable r in the amplified microphone currents. The advantage of an arrangement as here disclosed consists in that by the use of electrolytic condensers the operating direct current can bederived in a simple manner from the power pack without the 5 construction of the outfit becoming particularl costly or cumbersome.

Contrasted with battery voltage supply for the microphone, say, by the use of a dry battery or cell, the present arrangement ofiers the advantage 0 that the microphone whentheoutfit is connected, is ready foroperation at any time while, on the contrary, when the outfit is disconnected, no uri necessary consumption of current takes place.

Failure of the outfit onlaccount of exhaustion 5 of the microphone battery source is here, of

course, impossible.

. I claim: '1

1. An amplifier circuit having connected there- 5 nected to the alternating current source, one of a (Cl. 179-1) r the secondary windings of said transformer coning two electrolytic condensers connectedin par- 10' allel with said last-mentionedsecondary winding, 'said filter being connected to said microphone for supplying current thereto.

2. An amplifier circuit having connected therewitha-microphone, operated by current means energized from an alternating current source. said operating current means comprising a source of alternating current, a single transformer having a primary winding and a plurality of secondary windings, the primary winding of said transformerconnected to the alternating current source, one of the secondary windings of said transformer connected to a rectifier tube, a filter connected to said rectifier tube-and said secondary winding, said filter connected to said amplifier for supplying current thereto, at least one other secondary winding of said transformer connected to a dry copper oxide rectifier, a second filter connected to said dry copper oxide rectifier and said last-mentioned secondary winding, said sec- 0nd filter comprising-two electrolytic condensers connected in parallel with said last-mentioned secondary. winding, said filter being connected to said microphone for supplying current thereto.

3. An amplifier circuit having connected therewith a microphone operated by current means 1 energized from an alternating current source,

said operating current means comprising a source i of alternating current,'a single transformer having a primary anda plurality of secondary wind- 40 ings, the primary winding of said transformer connected to the alternating current source,'one of the secondary windings-of said transformer connected to a rectifier tube, a filter connected to a said rectifier tube and said secondary winding, ,5 said filter connected to said'amplifier for supplying current thereto, at least one other secondary winding of said transformer connected to a se lenium'rectifier, a second filter connected to said selenium rectifier and said last-mentioned.secondary winding. said second filter comprising two electrolytic condensers connected in parallel with said last mentioned secondary winding, said filter being connected to said microphone for supplying current thereto. I

4. An amplifier circuit having connected therewith a microphone operated by current means energized from an' alternating current source. said operating current means comprising a source of alternating current, a single transformer having a primary and a plurality of secondary windings, the primary,,-winding of said transformer connected to the alternating current source, one

of the secondary windings of said transformer connected to a rectifier tube, a filter connected to said rectifier tube and said secondary winding,

said filter connected to said amplifier for supplying current thereto, at least one other secondary winding, of said transformer connected to a rectifier, a second filter comprising electrolytic condensers, said filter connected to said rectifier and in parallel with said last-mentioned secondary winding, said second filter connected to said microphone for supplying current thereto.

5. An amplifier circuit having connected therewith a microphone operated by current means energized from an alternating current source, said operating current means comprising a source of alternating current, a single transformer having a primary and a plurality of secondary wind ings, the primary winding of said transformer connected to the alternating current source, one 7 a rectifier, a second filter comprising two electro- -lytic condensers of substantially 100 microfarads each connected to said rectifier and in parallel with said last-mentioned secondary winding, sald second filter connected to said'microphone for supplying current thereto.

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